Converting outboard to electric?

Discussion in 'Electric Propulsion' started by andysailor, Jun 18, 2017.

  1. andysailor
    Joined: May 2017
    Posts: 14
    Likes: 0, Points: 1
    Location: Sweden

    andysailor Junior Member

    What is the importance when thinking of converting an outboard to electric?
    Let say I put an 10kw BLDC motor on an OB. The electric runs 2000-6000 rpm. An OB at around 10-15hp should be able to handle the 10kw?
     
  2. gonzo
    Joined: Aug 2002
    Posts: 12,345
    Likes: 197, Points: 63, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    10kW = 7.5 HP. I think that the rpm range of the electric motor is not correct. It should show 0-max. rpm. Depending on the design of the motor, the torque/rpm curve will vary. The controller type may also affect it. Check what the maximum torque of the gas engine was, and that is what the shafts and gears are rated to. Ideally, you shouldn't exceed them.
     
  3. andysailor
    Joined: May 2017
    Posts: 14
    Likes: 0, Points: 1
    Location: Sweden

    andysailor Junior Member

    Thanks for reply!

    I can agree that it should say 0-6000 rpm. In the spec it says: 2000-6000 (customizable). It is the HPM-10KW I found here
    10kw is same a 7.5Hp, BUT, it´s not comparable with a 7.5hp OB cause of different efficiency. An OB's efficiency is much lower. A 10kw should equal about 20hp I think.
     
  4. gonzo
    Joined: Aug 2002
    Posts: 12,345
    Likes: 197, Points: 63, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    No, that is wrong. 10kW = 7.5 HP not 20HP. Power is the same regardless of source. If you use a windmill to produce 10kW, it will also be equivalent to 10kW from an electric motor or 7.5Hp from an outboard. There is a lot of misleading sales hype that is completely false about "electric horses" being bigger than "gas horses. It is absolutely wrong. As far as efficiency, what are you measuring? The power loss to create energy, the loss charging the battery, the internal heat loss during discharge plus the heat, friction and other losses at the motor need to be compared to the losses in an internal combustion engine to justify the claim.
     
  5. DCockey
    Joined: Oct 2009
    Posts: 3,820
    Likes: 127, Points: 63, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

    That conversion is backwards.

    1 HP (imperial) = 746.7 Watts = 0.7467 kW or 1 HP (metric) = 735 Watts = 0.735 Watts Horsepower - Wikipedia https://en.m.wikipedia.org/wiki/Horsepower For the purposes of this discussion the difference is not relevant. I'll use the imperial definition of HP.
    Inverting 1 kW = 1.34 HP so 10 kW = 13.4 HP

    The conversion above is of energy units, not the relationship between electric power in to mechanical power out for a motor. If the electric motor produces mechanical power of 10 kW then it will produce 13.4 HP. However if the 10 kW rating of the motor is electric power in then the mechanical power out will be less, with the ratio of mechanical power out to electric power in being the efficiency.

    Outboard motors are rated on the power at the propeller, which will be less than the power at output shaft of the gas engine. So the gas engine of a 15 HP outboard will produce more than 15 HP.

    So far we've only discussed power, not torque. However torque will be more important than power in determining whether the outboard lower unit will last with the electric motor.

    Any idea what the maximum torque of the electric motor is?
     
  6. andysailor
    Joined: May 2017
    Posts: 14
    Likes: 0, Points: 1
    Location: Sweden

    andysailor Junior Member

    Thank you guys!
    I shall focus more on torque from now on :D

    Here I found a torque diagram
     
  7. gonzo
    Joined: Aug 2002
    Posts: 12,345
    Likes: 197, Points: 63, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    Oops, I did convert backwards. I did point out that the torque should not be more than what the original gas engine had though.
     
  8. CDK
    Joined: Aug 2007
    Posts: 3,313
    Likes: 141, Points: 63, Legacy Rep: 1819
    Location: Adriatic sea

    CDK retired engineer

    If I understand your question correctly you want to remove the gasoline engine and put an electric motor there.
    The answer is yes, a 10hp outboard leg can easily handle a 10kw motor. The electric motor delivers its power much more gently, without vibration and with a much lower heat load. You can block the gear in fwd and reverse electrically.
     
  9. gonzo
    Joined: Aug 2002
    Posts: 12,345
    Likes: 197, Points: 63, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    You should use the gear shifter. Otherwise, if you keep it forward, the gears will not mesh properly and they will not be pushing against the thrust bearing. As for delivering the power more gently, the low rpm torque of an electric motor may be putting more stress.
     
  10. Barry
    Joined: Mar 2002
    Posts: 821
    Likes: 31, Points: 28, Legacy Rep: 158

    Barry Senior Member

    Torque of the electric motor is not really relevant in this situation (except as a component of horsepower) because it is the propeller that will determine the loading on the gears. The propeller will be able to create a value of torque
    dependant on its rpm which will feed back to engine. Ie you could have a 100 foot pound torque motor (at a specific rpm) BEING ABLE to create the 15 hp at that rpm or a 200 foot pound torque motor at the same rpm, but because the load is created from the prop, the torque produced by the higher torque CAPABABLE motor will be the same as the lower torque motor

    There has been a couple of threads over the years that some people are under the opinion that motors are rated on input horsepower, ie so many amps times volts (watts) in and you get a theoretical value of the motor then you adjust for efficiencies etc and come out with a lower output horsepower. I am not sure where this idea comes from but AndySailor appears to be a proponent of it

    In AC motors, when you buy a 1 hp motor, continuous duty, the output is 1 hp at its specified rpm. (certainly there are some slight variations as voltage even in modern day service and obviously some wire losses, but for all intents and purposes
    1 = 1
    NEMA, National Electrical Manufacturers Association, the standard that most US made AC motors (and perhaps DC) include relevant information on the tag that is attached to the motor

    DC can create a bit of different circumstances. Ie if you get a torque curve from a manufacturer, it most often is the maximum torque that the motor can put out at that rpm with excess current available. For the most part, you would not want to run a DC motor at this RPM and Load as it will normally overheat due to large current draws.
    What you need to do if you are matching a DC motor as a replacement in the OP's case is to get the motor manufacturer to supply a horsepower curve that is a continuous duty curve.

    Your comment, Andy, "I will focus more on torque now". You need to focus on hp available at the various rpm that you want to run the engine at.
    I can take a 1/16 horsepower motor and gear it right to create hundreds of pounds feet of torque but 1/16 hp will not run your boat
     
    Last edited: Jun 20, 2017
    Jed233 likes this.
  11. gonzo
    Joined: Aug 2002
    Posts: 12,345
    Likes: 197, Points: 63, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    A lot of the confusion come from the false advertising for electric motors. They claim their horses are bigger.
     
  12. Barry
    Joined: Mar 2002
    Posts: 821
    Likes: 31, Points: 28, Legacy Rep: 158

    Barry Senior Member

    Gonzo, do you think that the companies that make big claims are US manufacturers or from the East?
     
  13. gonzo
    Joined: Aug 2002
    Posts: 12,345
    Likes: 197, Points: 63, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    I think it is typical of the industry all over. They often show graphs and figures without scale, so they can't be compared. Also, they use thrust instead of power. If the thrust is bollard pull, the maximum speed could be 0.001 knot.
     
    Jed233 and kerosene like this.
  14. kerosene
    Joined: Jul 2006
    Posts: 972
    Likes: 48, Points: 28, Legacy Rep: 358
    Location: finland

    kerosene Senior Member

    I think the origin is from small trolling motors. They were propped for really sluw speed but nobody wanted to market a 0.25hp motor so they started using thrust instead.
     
    gonzo likes this.

  15. Matthew Lee Towne
    Joined: May 2017
    Posts: 139
    Likes: 1, Points: 18
    Location: Oviedo

    Matthew Lee Towne Senior Member

    Have you considered an AC motor? It will require a controller but should give you an almost flat power curve and plenty of torque. Not to mention plenty of RPM range. You might even be able to regenerate some power when slowing.
     
Loading...
Similar Threads
  1. alan craig
    Replies:
    9
    Views:
    1,321
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.